Abstract. The temperature and wind of the ECMWF ERA-40 reanalysis in the tropical
lower stratosphere during the period 1988–2001 has been evaluated by
comparison with independent in situ measurements of 21 IR Montgolfier
and superpressure long-duration balloon flights performed by CNES from
Pretoria (26° S) in South Africa in 1988–1989, Latacunga (1° S)
in Ecuador in 1991–1998 and Bauru (22° S) in Brazil in 2000–2001.
The ERA-40 temperature displays a bias varying progressively from +1.16 K
in 1988–1989, to +0.26 K in 1994–1996 and −0.46 K after 1998,
the latter being fully consistent with recent evaluations of ECMWF
operational analysis from radio occultation and in situ long-duration
balloon observations. The amplitude of the bias and its evolution are
very similar to the results of a previous evaluation from radiosondes
in 1991–2003, suggesting that the origin of the drift of ERA-40 might be
mainly due to errors in the series of satellite measurements of MSU,
replaced by AMSU in 1998, assimilated in the model.

The ERA-40 zonal wind speed in the lower stratosphere appears slightly
overestimated by 0.7–1.0 m/s on average in both the tropics and
equatorial region, that is by 5–10% compared to the average 10–20 m/s
wind speed. This bias, fully consistent with a recent evaluation of
ECMWF operational analysis in 2004, is found constant during the whole
1988–2001 period, suggesting
a shortfall in the variabililty of ERA-40
horizontal winds in the lower stratosphere in the tropics and the
equatorial region.
Finally calculated trajectories
using ERA-40, frequently used for analysing field observations, are found
in error compared to that of the balloons by ±500 km after 5 days
and ±1000 km after 10 days.